Many sub-sonic aircrafts, such as helicopters, use or will be using infrared optical sensing systems. In this application, a pod or turret containing an infrared sensor is mounted on the aircraft. Servo-mechanisms are used to rotate or otherwise articulate the pod to permit the sensor's field of view to be orientated in a variety of directions.
For protection from the airstream, the sensor is usually mounted behind the pod or turret flat window. The use of a flat window, however, creates a significant aerodynamic drag on the pod which make movement of the pod difficult at high speeds. Accordingly, the use of a curved rather than a flat window in this airborne optical application is very desirable from a reduced aerodynamic drag viewpoint. A reduction in the aerodynamic drag would not only reduce the torque needed to be produced by the pod servo-mechanism, but would also reduce the amount of fuel consumed by the aircraft. Additionally, a lower aerodynamic drag would reduce the mechanical vibrations on the pod or turret.
In spite of these mechanical advantages, the use of a curved window introduces difficult optical problems for the sensor. These problems include the optical power of the curved window and the aberrations of a window. One approach to solving these optical problems would be to try to have the sensor compensate for the optically induced aberrations in the curved window. However, this approach is undesirable because it will require complex and costly modifications to the sensor.
Another prior approach to solving these problems was the use of a curved window design consisting of a hemi-spherical germanium shell. This window possessed weak negative optical power, spherical aberration, coma and chromatic aberration. Additionally, when the optical axis of the sensor was positioned along a line which did not pass through the center of the hemi-spherical dome, the dome also introduced axial coma. While the use of germanium as the window could reduce the chromatic aberration to acceptable levels, correction of the defocus and remaining aberrations would still have to be accomplished through complex and costly design modifications to the sensor.
In contrast to the above, the present invention minimizes the aberrations of a curved window by utilizing certain changes to the hemi-spherical germanium shell of the window. These parametric changes compensate for the optical aberrations which would otherwise make it difficult for a sensor to see through the window.
These parameric changes are determined through a process which optimizes the optical design of the curved window. This process may conveniently utilize conventional optical design computer programs which will calculate the ray traces through the window. The object of the process is to minimize the bend in the ray aberrations as they pass through the window. Aside from basic information about the window, such as a starting inner radius of curvature, the rays themselves need to be defined. The directions for a set of meriodional and skew rays (e.g., 20) need to be defined over the field of view for the aperture of the sensor. The angles of the rays after they have passed through the window will be calculated. The variables from these calculations, such as the conic constant and the aspheric terms, can then be changed to minimize the differences between target and actual values for the ray traces using a damped least squares analysis. The terms which result from this procedure, such as the inner radius of curvature, the conic constant and the aspheric constants, can then be employed in an equation which will describe the window's surface profile for machining.